The document defines key terms related to the environment and ecology. It states that the environment includes natural and external conditions that influence living things. Ecology is defined as the study of how animals and people interact with their surroundings. Environmental sustainability is the long-term maintenance of ecosystems that allows them to continue for future generations. An ecosystem is defined as the complex assemblage of animals, plants, and their environment through which energy and materials move. The document also refers to the food chain/web as the process by which the sun feeds plants, plants feed herbivores, herbivores feed carnivores, carnivores die and benefit decomposers, decomposers feed the ground, and nutrients return to plants.